1. Field of the Invention
This invention relates to high-quality neutral-density filters and, in particular, to a neutral white-light filter constructed with a novel prism arrangement that is particularly suited for telescopic applications.
2. Description of the Prior Art
In telescopic solar astronomy, objects must be observed using optical filters that reduce the intensity of the light, especially at infrared and ultraviolet wavelengths. The human eye is particularly susceptible to damage from exposure to IR and UV wavelengths because they produce extremely harmful chemical and thermal effects on the retina, respectively. Therefore, when human observation of an incoming image is desired in an instrument such as a telescope, it is necessary to attenuate these wavelengths below acceptable levels. In the case of white light, attenuation by a factor of at least 10−5 is considered safe. Thus, the use of appropriate filters is extremely important for observation of very bright objects such as the sun
Neutral-density filters are defined as filters capable of attenuating a wavefront uniformly across the spectrum of visible light. In practice, these filters are implemented with combinations of optical components, each affecting a different spectral range, which therefore can only approximate neutral-density performance. In telescopes, where attenuation is desired throughout the entire white-light spectral range, it is usually obtained using a Herschel wedge in combination with additional filtering elements. As is well known in the art, a Herschel wedge operates in reflection, diverting about 5% of the incoming light toward the optical path of the observable beam, which is then further attenuated with filters to achieve a safe level for human observation. The transmitted beam, which constitutes about 95% of the incoming light, is either absorbed as heat by the telescope mass, typically in a refrigerated heat-sink component, or is dumped as an outgoing beam through an opening in the housing of the telescope.
Thus, the Herschel-wedge solution is unsatisfactory in at least two respects. From a functional viewpoint, the use of additional filters in the optical path of the observable beam to attenuate it to safe levels introduces undesirable spectral and optical non-uniformities; that is, the cumulative filter effect is not spectrally neutral and the quality of the image-forming wavefront is degraded. In addition, since most neutral-density filters use metallic layers (such as aluminum, silver or gold coatings) which heat up during use, the wedge assembly is not thermally stable. From an operational point of view, the fact that most of the energy of the incoming light has to be absorbed by the system increases the thermal-stability problems and requires expensive and cumbersome cooling units. When the transmitted beam is dissipated in the surrounding environment, the highly concentrated energy dumped by the beam represents a serious health hazard to anyone accidentally encroaching and blocking its path in the vicinity of the telescope.
Therefore, any filter capable of producing a neutral attenuation of a light beam across the white-light spectrum (for the purposes of this disclosure defined as the spectral range from about 200 to about 1,600 nm) and operable without excessive thermal effect would represent a very desirable advance in the art. This invention achieves these goals with optical filters based on a variety of prism arrangements adapted to split the incident light into a useful portion directed toward the observer (or image-plane surface) and a rejected portion reflected toward the incoming beam.